This invention relates to wireless Local Area Networks (wireless LANs) and more specifically to the area of dynamic channel assignment for wireless LANs. Wireless protocols, such as the 802.11 family of protocols for WLANs, make use of a finite number of fixed channels. Each channel is a distinct communication path between an access element and a client remote element, such as a mobile station. Each access element must be assigned a specific fixed channel for operation, and all neighboring access elements must avoid using the same channel in order to minimize interference. The task of assigning channels to access elements in a network is also known as ‘graph coloring,’ (after the corresponding mathematical concept) where each access element represents a node and each color represents a specific channel. When two neighboring access elements use the same channel (or partially spectrally overlapping channels where such is possible), interference occurs that significantly degrades communication performance.
An optimally configured wireless network ensures that all potentially interfering (neighboring) access elements are configured to minimize the possibility of channel duplication. However, certain wireless protocols (e.g., 802.11b) provide for only a small number of spectrally non-overlapping channels assignable to the access elements, depending for reliability of communication more on the ad hoc lack of density of access elements and low levels of activity in a limited region of propagation. Thus it is impossible to ensure that neighboring access elements do not use duplicate channels in denser, physically realizable networks. With such technologies, other factors must be used to assist in the management of channel assignment.
Because of minimal interference in licensed bands, prior art in licensed spectrum cellular networks has dealt with the frequency reuse problem as a two-dimensional color map problem, ignoring interference as a factor. The nature of the interference sources in the unlicensed bands and the frequent changes in the environment of wireless LAN systems requires that the wireless channel assignment techniques be highly dynamic to maintain optimal performance. Additionally, the subscriber load of such a network must be taken into account to ensure that the best channels are allocated where needed.
Known WLAN products (such as those marketed by Cisco Systems) have a dynamic channel selection feature. Those WLAN products ‘listen’ for traffic on each of the channels available and then choose the one with the least amount of detected traffic during the period of the selection process. However, such methods of channel selection are made with limited local information and do not take into account the needs of neighboring access elements. In a fully distributed WLAN network, access elements make channel selections independent of each other, which leads to suboptimal use of channels as network resources.
By their licensed nature, cellular systems are subject only to self interference so that by definition all channels are treated equally. An example of a patent which describes dynamic channel assignments in a cellular system is U.S. Pat. No. 5,749,044 entitled “Centralized Dynamic Channel Assignment Controller and Methods.” A further example of automatic channel assignment in a cellular system is U.S. Pat. No. 6,175,739 entitled “Channel Selection Method of a Wireless Communication System.” The systems therein described fail to address the interfering channel problem.
Conventional networking design isolates and modularizes various communication tasks into layers. The ISO model calls for a physical layer, a media access control layer and a routing layer, as well as an application layer. The model does not provide for ready visibility of activities across layers and, particularly, visibility of activities in other than neighboring layers.
There are known solutions to the graph coloring problem using various algorithmic approaches. One such approach is called the “greedy” approach wherein the most important nodes are “colored” or assigned first and then less-important nodes are subsequently colored or assigned. Representative references are D. Brelaz, “New Methods to Color the Vertices of a Graph,” Communications of the ACM, vol. 22, no. 4, April 1979, pp. 251-256 and J. Randall-Brown, “Chromatic Scheduling and the Chromatic Number Problems,” Management Science, vol. 19, no. 4, December 1972, pp. 456-463. Work has been done in this field at Carnegie-Mellon University to solve problems related to static assignment of radio channels to base stations based on overlapping coverage areas. This work has not been published but is the subject of a prior-filed patent application in the names of Hills and Schlegel assigned to Carnegie-Mellon University and will be considered prior art in due course.
What is needed is a technique which takes into account interference from other competing systems and inband (industrial) devices, and network traffic load, and which recognizes that the available channels are not of equal priority in a dynamic environment.